The eastern branch of the East African Rift System developed over the past 20 million years (Ma), with uplift and large-scale faulting manifested since the early Miocene south of the Afar Triangle (Ethiopia) and predominantly during the late Miocene farther south (Partridge, 2010). Tectonic faulting and volcanism in the southern Kenya rift is younger – predominantly Pliocene and Pleistocene – with the formation of sedimentary basins mainly during the early and middle Pleistocene. The oldest of these basins in which intensive geological, paleontological, and paleoanthropological study has been conducted is Olorgesailie (Figure 30.1; e.g., Isaac, 1977, 1978b; Shackleton, 1978; Potts, 1989, 1994, 2007; Deino and Potts, 1990; Behrensmeyer et al. 2002; Potts et al., 2018). The mammalian fauna of the Olorgesailie Basin is the focus of this chapter. Several other depocenters to the south and west of Olorgesailie, including west of Lake Magadi, have produced less-extensive middle Pleistocene mammalian fossil discoveries (e.g., Lainyamok, Oldonyo Nyokie, Lenderut); the faunas from these sites will also be discussed briefly here.